The broad,long-term objective of this proposal is to develop a detailed understanding of structure-function relationships within the voltage- dependent sodium channel molecule. The Specific Aims are: 1) To clarify the functional significance of each of the four S4 segments within the tetrameric sodium channel molecule; 2) To clarify the interactions between activation and fast inactivation at the structural level; 3) To clarify the effects of "distant" sites on activity of the S4 segments within each domain. The experimental design involves detailed functional analysis of mutant channels expressed in Xenopus oocytes following site directed mutagenesis to alter specific regions of the channel molecule. The details of this design reflect our sophisticated approach to electrophysiological analysis of sodium channel mechanisms and our recent advances in this area. The methods used involve voltage-clamp analyses of channel properties (in which we have much expertise) and molecular biological methods which are relatively new to our laboratory. We have recruited a coinvestigator from this campus with extensive experience in site-directed mutagenesis and have developed collaborative agreements with other investigators to ensure appropriate support for this initiative. Our proposed work is strongly health-related since the sodium channel molecule is affected by many drugs and is the basis for the electrical excitability of cardiac muscle, skeletal muscle and nerve cells.